1. Field of the Invention
The present invention relates to a filter catalyst for purifying exhaust gases, such as those emitted from diesel engines and including particulates.
2. Description of the Related Art
Regarding gasoline engines, harmful components in the exhaust gases have been reduced securely by the strict regulations on the exhaust gases and the technological developments capable of coping with the strict regulations. However, regarding diesel engines, the regulations and the technological developments have been advanced less compared to those of gasoline engines because of the unique circumstances that the harmful components are emitted as particulates (i.e., particulate materials, such as carbonaceous fine particles, sulfuric fine particles like sulfates, and high-molecular weight hydrocarbon fine particles, hereinafter collectively referred to as “PMs”).
As exhaust-gas purifying apparatuses having been developed so far for diesel engines, the following have been known. For example, the exhaust-gas purifying apparatuses can be roughly divided into trapping (or wall-flow) exhaust-gas purifying apparatuses and open (or straight-flow) exhaust-gas purifying apparatuses. Among these, clogged honeycomb structures made from ceramic (i.e., diesel PMs filters, hereinafter referred to as “DPFs”) have been known as one of the trapping exhaust-gas purifying apparatuses. In the DPFs, the honeycomb structures are clogged at the opposite openings of cells in a checkered manner alternately, for instance. The DPFs comprise inlet cells clogged on the downstream side of exhaust gases, outlet cells neighboring the inlet cells and clogged on the upstream side of the exhaust gases, and filter cellular walls demarcating the inlet cells and the outlet cells. In the DPFs, the exhaust gases are filtered by the pores of the filter cellular walls to collect PMs.
In the DPFs, however, the pressure loss increases as PMs deposit thereon. Accordingly, it is needed to regularly remove deposited PMs to recover the DPFs by certain means. Hence, when the pressure loss increases, deposited PMs have been burned with burners or electric heaters conventionally, thereby recovering the DPFs. However, in this case, the greater the deposition of PMs is, the higher the temperature increases in burning deposited PMs. Consequently, there might arise cases that the DPFs are damaged by thermal stress resulting from such burning.
Hence, continuously regenerative DPFs have been developed recently. In the continuously regenerative DPFs, a coating layer comprising alumina is formed on the surface of the filter cellular walls of the DPFs, and a catalytic ingredient such as platinum (Pt) is loaded on the coating layer. In accordance with the continuously regenerative DPFs, since the collected PMs are oxidized and burned by the catalytic reaction of the catalytic ingredient, it is possible to regenerate the DPFs by burning PMs simultaneously with or successively after collecting PMs. Moreover, since the catalytic reaction occurs at relatively low temperatures, and since PMs can be burned when they are collected less, the continuously regenerative DPFs produce an advantage that the thermal stress acting onto the DPFs is so less that the DPFs are inhibited from being damaged.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 9-220,423 discloses such a continuously regenerative DPF whose filter cellular wall exhibits a porosity of from 40 to 60% and an average pore diameter of from 5 to 35 μm, and whose coating layer is formed of a porous oxide. In the porous oxide, particles whose particle diameter is less than the average pore diameter of the filter cellular wall occupy 90% by weight or more. When such a porous oxide with a large specific surface area is coated on DPFs, it is possible to form the coating layer not only on the surface of the filter cellular walls but also on the inner surface of the pores. Moreover, when the coating layer is coated in a fixed amount, it is possible to make the thickness of the coating layer thinner. Accordingly, it is possible to inhibit the pressure loss from enlarging.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 6-159,037 discloses a continuously regenerative DPF whose coating layer is further loaded with an NOx-sorbing member. With the arrangement, NOx can be sorbed in the NOx-sorbing member. Consequently, when a reducing agent such as light oil is sprayed onto the coating layer, it is possible to reduce the sorbed NOx to purify.
However, at the inlet end surface of DPFs, the openings of the inlet cells and the clogged outlet cells exist in a neighboring manner. Therefore, the opening ratio is so small that it is 50% or less at the inlet end surface. Accordingly, there arises a disadvantage that PMs and ashes are likely to deposit on the clogged outlet cells. Moreover, when DPFs are operated continuously under the condition that the inlet gas temperature is low, or when a reducing gas is kept to be sprayed to reduce the NOx sorbed in the NOx-sorbing member, the layer of the deposited PMs and ashes has grown from the clogged openings of the outlet cells to the openings of the inlet cells to close the openings of the inlet cells. Consequently, the back pressure might increase to lower the output power of diesel engines. In particular, when a reducing agent such as light oil is sprayed, the openings of the inlet cells are more likely to be closed because the liquid particles collide with the inlet end surface of DPFs directly.
Moreover, continuously regenerative DPFs have a problem with the limited activity. Specifically, it is impossible to increase the loading amount of catalytic ingredient because the coating amount of the coating layer is limited in view of the pressure loss. On the other hand, when a large amount of catalytic ingredient is loaded on a thin loading layer, the loading density of catalytic ingredient is enlarged so that the granular growth of catalytic ingredient occurs at high temperatures. As a result, continuously regenerative DPFs are deteriorated in terms of the durability.
Hence, as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 9-032,539, for example, it is possible to think of disposing a straight-flow oxidizing catalyst on an upstream side with respect to DPFs. With such an arrangement, the oxidizing catalyst oxidizes gaseous hydrocarbons (HC), carbon monoxide (CO) and liquid soluble organic fractions (SOF), and further turns NO into NO2 which is then sorbed in an NOx-sorbing member. Accordingly, the exhaust-gas temperature increases so that the conversions of PMs and NOx are improved. Moreover, in DPFs, the oxidizing catalyst turns reducing agents into gas. Consequently, liquid particles do not collide directly with the inlet end surface of DPFs. Therefore, it is possible to inhibit the openings of the inlet cells of DPFs from being closed.
However, most of PMs pass through the oxidizing catalyst as they are. Accordingly, PMs have deposited on the inlet end surface of DPFs in not a small amount. It is impossible to fundamentally solve the problem. Moreover, when a casing accommodating DPFs therein is limited in terms of the length, it is necessary to shorten the length of the oxidizing catalyst or DPFs. However, the oxidizing catalyst or DPFs with a shortened length cannot be assembled with casings with secured accuracy, and suffer from poor reliability in terms of the strength.